The present invention relates to optical sources, and in particular to light emitting diode (LED) light sources. It is particularly applicable to lighting applications where an LED is contemplated for use as an approximation to a point or line light source, or as an approximation to an extended light source which emits with approximately uniform intensity over an extended solid angle. However, the invention is not so limited, and may find application in other situations where a particular external light emission intensity distribution is required. Also with respect to scope, in the following disclosure the word xe2x80x9clightxe2x80x9d is to be broadly interpreted to include any applicable spectral range including but not limited to visible, ultraviolet, and infra-red radiation.
LED""s have a number of advantages as light sources, such as relatively cool operating temperatures, high achievable wall plug efficiencies, and a wide range of available emission colors extending throughout the visible and into at least the adjacent infra-red and ultraviolet regions dependent upon the choice of semiconductor material. However, LED""s have some disadvantages as well, such as poor light coupling through the LED surface which reduces external quantum efficiency, and a highly directional external intensity distribution.
Because of the relatively large refractive index of most LED materials (refractive index n greater than 3 in most cases), internally generated light rays incident upon the LED surface at angles greater than about 20xc2x0 away from the surface normal experience total internal reflection and do not pass through the LED surface. It is known in the prior art to improve external light coupling during LED packaging through the use of a transparent encapsulant typically in the shape of a hemispherical dome. The encapsulant material is usually an epoxy resin or the like, with a refractive index nxcx9c1.5. The encapsulant serves the dual purposes of improving light coupling by reducing total internal reflection losses, and hermetically sealing the LED die.
Although hemispherical dome encapsulation improves LED external light coupling efficiency, it does not significantly change the typically highly directional intensity distribution. An LED is typically an essentially Lambertian source in which the light intensity varies approximately with the cosine of the angle away from the LED surface normal. This intensity distribution strongly enhances light intensity in the forward direction, making the LED a highly directional light source. In contrast, the filament of an incandescent light bulb emits with essentially similar intensity at most viewing angles, and is a reasonable approximation to a point light source, or to a line light source in the case of a longer filament. Therefore, direct replacement of an incandescent source by an LED in a lighting system usually results in very inefficient usage of the LED emission. For example, the parabolic reflector of a flashlight is designed to work with approximately point light source such as an incandescent bulb filament, and does not operate properly on the more directed LED emission distribution.
The prior art does not teach a method for packaging an LED in a manner which produces an external emission intensity distribution that more closely approximates a point or line light source. The present invention contemplates an improved LED package and LED packaging method which overcomes these prior art limitations and others.
In accordance with one aspect of the present invention, a light emitting diode (LED) package is disclosed. A transparent encapsulant surrounds the LED die. A reflective surface is disposed on the encapsulant surface essentially opposite the LED die surface.
In accordance with another aspect of the present invention, a method for manufacturing a light-emitting diode (LED) package which emits light in an essentially non-directional manner over at least a predetermined solid angle is disclosed. An LED die is mounted to a lead frame and electrically connected to the leads of the lead frame. At least the LED die is encapsulated in a transparent encapsulant. A reflective coating is applied to a portion of the encapsulant essentially opposite the LED die.
In accordance with another aspect of the present invention, a light emitting diode (LED) capsule is disclosed. An LED die is mounted within a lead frame and electrically connected thereto. A transparent encapsulant encapsulates the LED die and at least a portion of the lead frame. A reflecting surface is disposed on a portion of the encapsulant outer surface. Preferably, the reflecting surface is disposed essentially opposite to the light-emitting surface of the LED die. The capsule preferably further includes a refracting surface which cooperates with the reflecting surface to convert the distribution of the LED light emission intensity into a preselected external light emission intensity distribution. The refracting surface is preferably a portion of the encapsulant outer surface which has a preselected curvature.
In accordance with yet another aspect of the present invention, a light emitting diode (LED) capsule for producing an approximate extended light source with essentially uniform intensity distribution over an extended solid viewing angle is disclosed. An LED die is mounted onto a lead frame and electrically connected thereto. A transparent encapsulant encapsulates the LED die and at least a portion of the lead frame. A roughened surface is disposed on at least a portion of the encapsulant outer surface. Preferably, the roughened surface is a roughened depression in the encapsulant positioned essentially opposite the LED die, and is either filled with a reflective filling disposed within the depression and essentially conforming with the roughness of the roughened surface, or is coated with a reflective coating material which is disposed upon at least a portion of the roughened surface.
One advantage of the present invention is that it provides an LED package with a preferred external light emission intensity distribution.
Another advantage of the present invention is that it provides an LED package which outputs an approximate point light source distribution and is suitable as a replacement for an incandescent light bulb.
Another advantage of the present invention is that it adds only a single manufacturing step over conventional LED packaging, and that additional step is application of a reflective coating which may be realized using any of a number of established methods.
Another advantage of the present invention is that it is implemented at the package level, rather than as an add-on, thereby reducing costs and increasing performance.
Yet another advantage of the present invention is that it provides an LED package which generates an apparent extended light source with an approximately uniform intensity distribution over an extended solid viewing angle.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.